Unless a boundary condition is specified on interior boundaries (such as a contact resistance condition), the COMSOL Multiphysics software ensures continuity in the field variables across interior boundaries. For assembly geometries with identity pairs, select a Continuity node in the Pairs menu in the boundary part of the context menu for most physics interfaces. The Continuity condition is only suitable for pairs where the boundaries match.

Select the pairs where you want to impose continuity across the pair boundaries by clicking the Add button () and choose the pairs to add from the list in the Add dialog box that opens; then click OK.